1. Field of the Invention
The present invention relates to a carrier for use in development of an electrostatic latent image and, more particularly, to a carrier for use in the development of an electrostatic latent image coated with a thermoplastic resin which ensures a prolonged use and a stable charging ability.
The present invention also relates to an image forming apparatus using a two-component developer as a developer, and, more particularly, to an image forming apparatus using the above carrier for use in the development of an electrostatic latent image.
2. Description of the Prior Art
In electrophotography, a "magnetic brush process" has seen extended use in developing an electrostatic latent image and, as the developer used therefor, a two-component developer comprised of a mixture of magnetic carrier particles and toner particles has been widely used.
One of the drawbacks of the two-component developer is that toner particles are liable to become fused to the carrier surface, namely, the production of spent toner. It is known that spent toner causes fog and lowers the charging ability of a carrier and image density. Specifically, the spent toner is the phenomenon that toner particles adhere to and deposit on the surface of a magnetic carrier in a film form. The constituent of the magnetic carrier surface becomes similar to the toner constituent, causing a frictional charge, and failing to obtain a predetermined charging ability. Thus, upon the occurrence of spent toner, the magnetic carrier needs to be discarded and then replaced with a new magnetic carrier.
To solve the above problem, there has been proposed a variety of resin-coated carriers in which the carrier particle surface is coated with various resins. In view of resistance to surface contamination, the use of fluorocarbon polymers have been proposed. JP-A-314198/1996 proposes to use, as a coating resin, polyolefin having a specific weight-average molecular weight (Mw), and number-average molecular weight (Mn), in order to avoid adhesion of toner, etc. This has some measure of success in preventing adhesion of toner particles and the like.
With the former means, however, fluorine itself has a strong chargeability of negative property. Accordingly, the charging ability of toner into negative property is low, thereby the use thereof is restricted. With the latter means, the developing property after a prolonged use is not yet fully satisfied.